Brian R. Fogg

Fiber optic impact detection and location system embedded in a composite material

We present an impact detection and location system which uses fiber optic extrinsic Fizeau interferontric sensors embedded in a graphite/epoxy composite laminate. The acoustic signals generated by the impact events are detected by four fiber optic sensors. The fiber sensor and the embedding process are described. Also developed are a mathematical method and computer program that allow calculations of unambiguous impact location from the sensor data. The impact location can be determined with a 0.5 millimeter resolution and an accuracy typically less than five millimeters.

Embedded Fabry-Perot fiber optic strain sensors in the macromodel composites

We introduce the use of fiber optic strain sensors embedded within a macromodel composite employed to study and validate micromechanical theories. Fabry-Perot (FP) fiber optic strain sensors (FOSSs) embedded within a macromodel composite are shown to be an accurate and precise means of making local internal strain measurements in and around damage events. The measurements made by the sensors compare closely to those obtained from resistance strain gauge data verified through presently accepted micromechanics. The optical strain sensors effectively measure both the signature of fiber fracture and the resulting strain concentration due to the damage event. The sensors add a new dimension to the validation and development of micromechanics. The approach assists in the formulation of new concepts for interpretation and prediction of actuator/sensor response in smart materials.

Polarimetric analysis of two-mode, elliptical-core optical fibers

We show that linearly polarized light launched at different angles with respect to the major axis of a two-mode, elliptical-core fiber results in an amplitude-modulated beat-length pattern in the far field. The variation of the output pattern is explained theoretically and confirmed experimentally. A polarizer placed at the output of the fiber is shown to eliminate the amplitude modulation in the output signal. A simple theoretical model, based on the weakly guiding assumption, is shown to be helpful in determining few-mode fiber sensor parameters in practical systems.

Spatially weighted, grating-based, two-mode, elliptical-core optical fiber vibration sensors

Photoinduced refractive-index changes in two-mode, elliptical-core optical fibers affect the beat length and the sensor sensitivity. Chirped gratings are written by attaching such fibers to cantilever beams positioned in a strained state. We show that fibers with in-line chirped gratings, with the chirp being shaped in the form of a vibration-mode shape, can be used as spatially weighted fiber sensors for vibration analysis. We demonstrate enhanced detection of the first and second modes of vibration of a cantilever beam using this process; vibration mode suppression of the order of 10 dB is obtained.

Performance of extrinsic Fabry-Perot optical fiber strain sensors in the presence of cyclic loads

An experimental study was conducted to determine the utility of in-line optical fiber-based extrinsic Fabry-Perot interferometers (EFPIs), in a fatigue environment typical of aircraft structures. Metallic and composite coupons with EFPIs attached to and embedded within were tested in constant amplitude cyclic fatigue at room temperature. An additional composite coupon was tested similarly at an elevated temperature. For the consideration of composite material applications the objectives were to determine the durability of the sensor and its ability to measure strains accurately, even when the EFPI sensor was embedded at an angle with respect to the principal adjacent reinforcing fibers of the composite. For metals, in addition to durability considerations, research was conducted as to how the EFPI sensor may be used to detect crack initiation and growth. The results of the test program have established the excellent durability of the EFPI sensor element for fatigue loading up to 50,000 cycles at R equals 0.1 (tension-tension fatigue) with a maximum strain level of 3,500 microinch/inch, for both attached and embedded sensors, once the optical fiber and sensor survived the composite laminate panel curing process.

Photofluidics for integrating fiber sensors with high-authority mechanical actuators

An integration of fiber sensor technology and photofluidic interfaces is proposed. A practical implementation of such an integrated system is described in which sensing is performed by employing an acceptable fiber optic method while the logic and feedback is performed using existing fluidic methods. The proposed closed-loop system needs no electrical conversion or feedback information. The capability of the integrated system to accurately track the vibrational behavior of a beam is demonstrated.

Implementation and evaluation of an embedded extrinsic Fabry-Perot fiber optic strain rosette sensor

In this study three extrinsic Fabry-Perot interferometers are arranged in a rosette configuration and embedded in a homogeneous neat resin and a composite material to measure an arbitrary in-plane strain field. The data obtained from the embedded optical gauges are compared to external resistance strain gauges to assess the accuracy of the internal sensors. Tests are also conducted to evaluate the degradation in compression strength due to the presence of the optical sensor. Data suggests that the embedded rosette configured fiber optic sensor degrades the composites compressive strength by approximately 65%. The measured strength reduction is compared with classical models to predict this value.

Two-mode elliptical-core weighted fiber sensors for vibration analysis

Two-mode, elliptical-core optical fibers are demonstrated in weighted, distributed and selective vibration-mode-filtering applications. We show how appropriate placement of optical fibers on a vibrating structure can lead to vibration mode filtering. Selective vibration-mode suppression on the order of 10 dB has been obtained using tapered two-mode, circular-core fibers with tapering functions that match the second derivatives of the modes of vibration to be enhanced. We also demonstrate the use of chirped, two-mode gratings in fibers as spatial modal sensors that are equivalents of shaped piezoelectric sensors.

Optical fiber sensors for active structural acoustic control

We present the first demonstration of active acoustic control using optical fiber sensors. The fiber sensors are oriented symmetrically about the vertical centerline of a thin, simply supported baffle plate and are configured for the most effective detection of the odd-odd plate modes. The output from the fiber sensor is used as an error signal, a least-mean-square algorithm is used for digital processing, and control is achieved using a single piezoelectric actuator. Preliminary results obtained from the use of elliptical-core, two-mode fiber sensors attached to the plate show effective acoustic attenuation of 25 dB in on-resonance cases and 10 dB in off-resonance cases.

Spatially-weighted Vibration Sensors Using Tapered Two-mode Optical Fibers

Two-mode elliptical-core (e-core) fibers have been used as efficient vibration sensors when operated in the linear region.1 Such ruggedized e-core sensors can perform as vibration-mode filters when placed appropriately along the vibration antinodes of the beam.2 We report the development of distributed modal sensors using optical fiber techniques. The variable sensitivity of the fiber sensors has been achieved by utilizing the feature that the differential propagation constant in a two-mode fiber is directly dependent on the normalized frequency (or, V-number). Tapering the fiber changes the V-number and hence can change the sensitivity of the sensor along its length. We show that these sensors are fiber optic analogs of shaped, piezo-electric modal sensors that have emerged recently in the area of structural control3 and demonstrate their applications for clamped-free onedimensional beams.